Such an impact sensor is known from German Patent Publication DE-OS 2,207,831. The impact sensor is arranged in a sensor housing and serves for triggering a passive safety device when the vehicle impacts on an obstacle. The triggering of the safety device involves, for example, inflating an air cushion or the pulling tight of a safety belt. In the sensor housing there are arranged an acceleration sensor, an evaluating circuit for the electrical output signals thereof, a trigger circuit for triggering the passive safety device and a testing circuit for testing the ability of the impact sensor to function. The testing circuit produces an impulse train, whereby the impulse amplitudes are so dimensioned that in the evaluating and triggering circuit given threshold values can be checked. The testing impulses are caused to pass through the entire circuit and they are evaluated in the testing circuit. During the testing of the impact sensor to see whether it is able to function, the trigger circuit is separated from the passive safety device.
In the known sensor the acceleration system is a piezoelectric crystal, the signal of which are integrated once or twice depending on whether the triggering criterium is selected as a certain critical speed or a certain critical distance. The mentioned testing impulses are thereby directly applied to the electrodes of the piezoelectric crystal. The testing circuit is switched on at least prior to the beginning of a trip and preferably also at regular time intervals during the trip. The ability to function can be indicated, for example optically. For example, a light emitting diode could blink for a certain time after starting during the operation of the ignition of the vehicle. The diode could extinguish when the sensor is functional. If, during a testing it is ascertained that the sensor is not functioning properly, then, for example, a continuous blinking could be triggered.
In the known testing circuit only the electrical and electronic components of the impact sensor are tested. A further testing, for example, of a mechanical type is not provided.